This invention relates to protective relays used in electric power system applications and, in particular, to the automatic and/or semi-automatic control of these protective relays.
As shown in FIG. 1, in the transmission and distribution of electrical power within an electrical power system operated by an electric utility, there exists multiple substations (A, B, C, D) which control the switching and distribution of electric power from a generating source (main dispatch center) to various distribution points and loads within the power system. These substations normally include a plurality of power transformers whose primary windings are connected to input lines and whose secondary windings are connected to output feeders/distribution lines. The substations also include a plurality of circuit breakers which are coupled to the incoming transmission lines, the power transformers and the outgoing feeders to switch (and hence protect) the respective lines and/or piece of equipment when a fault (e.g., an overload) condition is sensed. The switching (opening and closing) of the circuit breakers within a substation is controlled by protective relays which, by controlling the opening and closure of the circuit breakers, protect the incoming transmission lines, the power transformers, the outgoing feeders and the circuit breakers contained within the substation. The protective relays may be of various configurations, including electromechanical, non-programmable static, and programmable microprocessor based relays. Each protective relay is designed such that when it senses, or responds to, a fault condition, it causes its associated circuit breaker(s) to interrupt the power to, or out of, the device the relay is designed to protect.
In general, the protective relays are designed to sense and/or respond to a fault condition based on a plurality of settings maintained in, or applied to, each relay. Those protective relays which include a microprocessor/controller normally also include memory storage in which these settings are stored. The stored settings may be compared to various signals and preset conditions to enable a relay to sense the presence of faults based on these different signals and preset conditions. Further, these microprocessor based relays have means for selecting one of the stored setting groups when a signal is presented.
Typically, the power ratings of pieces of equipment such as the power transformers, the power distribution lines and the circuit breakers change as a function of temperature. By way of example, a particular power transformer may be rated to safely carry 15,000 amperes at minus (-) 35 degrees centigrade and only 10,000 amperes at plus (+) 85 degrees centigrade. In general, the resistance of the wiring and material used to form the transformers and distribution lines increases with increasing temperature resulting in a decrease in the power rating of these pieces of equipment with increasing temperature. For the example of the particular power transformer, above, a protective relay associated therewith would be, for example, set to "trip" an associated circuit breaker when the current through the transformer would exceed 12-to-13,000 amperes in the winter and 8-to-9,000 amperes in the summer. Therefore, it is a normal operating practice for electric power utilities to change the settings of protective relays at certain times during the course of a year to take into account different seasonal ambient conditions such as temperature. It is also accepted practice in the industry to send a highly skilled technician to each substation to reprogram each protective relay or manually set a new setting group via a rotary switch in the substation. Because of the number of substations, the distance between them, and the time to re-set and test the various protective relays, this is both expensive and impractical.
Also, on a particular day, the ambient temperature in a substation can be substantially higher (or lower) than expected, which either leaves the settings of the various protection relays too sensitive (e.g., too low) or not sensitive enough (e.g., too high). This can lead to mis-operation of these protection devices which results in unwanted blackouts, or to a catastrophic failure of a power transformer or circuit breaker.
Thus, the fixed settings applied to the relays does not solve the problem resulting from the fact that the temperature within the substation can vary over a wide range at any instant of time. This gives rise to a basic problem since the settings of the relays within the substation and their set points is based on preset or preprogrammed information and not on the actual value of the temperature and load conditions. Because of the significant change in the power rating of the transformers and circuit breakers (and other pieces of equipment) as a function of temperature, undervoltage and overvoltage conditions can occur. Also, either there may be a failure to recognize a fault condition or a false error condition may be sensed resulting in unnecessary disruption of power to customers.